This invention generally relates to heat exchangers and more particularly relates to a system for removing sludge from sludge-bearing fluid flowing in a nuclear steam generator so that the sludge is not present therein in sufficient quantities to corrosively attack components located in the steam generator.
Before discussing the problem of corrosion in nuclear steam generators, it is advisable to first consider the structure and operation of a typical nuclear steam generator. In this regard, a typical nuclear steam generator comprises a vertically oriented shell and a plurality of inverted U-shaped tubes disposed in the shell so as to form a tube bundle. A tube sheet supports the vertical portions of the inverted U-shaped tubes at their lower ends. One lower end communicates with a primary fluid inlet chamber located beneath the tube sheet and the lower end communicates with a primary fluid outlet chamber also located beneath the tube sheet. The steam generator further comprises a cylindrical wrapper sheet disposed between the tube bundle and the shell to form an annular downcomer chamber, the wrapper sheet terminating a predetermined distance above the tube sheet. Moreover, the tube sheet may have a shallow cavity in an untubed central region thereof.
During operation of the steam generator, radioactive primary fluid (e.g., water), having been heated by circulation through a radioactive nuclear reactor core, enters the primary fluid inlet chamber and flows through the tube bundle and out through the primary fluid outlet chamber. At the same time, non-radioactive secondary fluid or feedwater flows down the annular downcomer chamber and impinges the tube sheet, the feedwater from the downcomer flowing radially inwardly along the tube sheet and upwardly among the tubes inside the wrapper. The feedwater then circulates around the tubes above the tube sheet in heat transfer relationship with the outside surfaces of the tubes, so that a portion of the feedwater is converted to steam. The steam is then circulated through a turbine connected to an electrical generator for generating electricity in a manner well known in the art.
As discussed hereinbelow, contaminants may be present in the feedwater in the form of suspended particulate material. This particulate material may settle-out of the feedwater and corrosively attack and damage the tube walls such that the radioactive primary fluid flowing in the tubes will eventually commingle with the non-radioactive secondary fluid flowing around the tubes. Commingling the radioactive primary fluid with the non-radioactive secondary fluid is undesirable because such commingling may compromise the safety of the nuclear power plant. The particulate material, which may be in the form of iron oxides and copper compounds together with trace amounts of other metals and/or metal oxides or even nonmetallic constituents, tends to settle-out of the feedwater onto the tube sheet in those areas of the tubesheet where the velocity of lateral flow across the tube sheet is insufficient to prevent settling. Settling of the particulate material onto the tube sheet can be harmful because it may create buildups of sludge deposits which may act as sites for concentrated corrosive agents. As stated hereinabove, these corrosive agents may corrosively attack and damage the tube walls and eventually lead to the undesirable commingling of the radioactive primary fluid with the non-radioactive secondary fluid.
Regions of low lateral or radial flow velocity across the tube sheet may lead to the settling-out of the particulate material to create the sludge that will accumulate on the tube sheet. In order to minimize corrosion tube damage caused by sludge buildup in such low velocity areas, it is desirable to localize this sludge buildup to the regions of the tube sheet where there are no tubes (e.g., along an untubed center lane or at the untubed central region of the tube sheet). Thus, it is desirable to control the flow of secondary fluid so that regions of low lateral or radial velocity occur in those regions of the tube sheet that have no tubes. Therefore, it is preferable to obtain the smallest low velocity area and, at the same time, locate it at the center region of the tube sheet where there are no tubes. Locating the low velocity area at the center region of the tube sheet tends to localize the area of sludge build-up to the untubed center region of the tube sheet rather than at regions of the tube sheet that have tubes.
It will be understood that the feedwater enters the tube bundle from beneath the tube wrapper, the radial inwardly flow along the tube sheet is impeded by the presence of tubes. Moreover, because there are no tubes along an untubed center lane of the tube sheet, there tends to be relatively high flow velocity therealong, thereby causing relatively low flow velocity in the regions of the tube sheet where tubes are present. To minimize center lane flow velocity and thereby increase the flow velocity in the regions of the tube sheet where tubes are present, center lane blocks are provided at spaced-apart locations along the center lane to inhibit the flow of secondary fluid or feedwater therealong. However, the center lane blocks do not eliminate the presence of other low velocity regions in the tube bundle. In this regard, when the feedwater enters the tube bundle from the bottom of the wrapper near the tube sheet, the radial inwardly flow tends to immediately turn upward because the flow resistance in the vertical direction, parallel to the tubes, is now much less than that in the lateral or radial direction. This results in low velocity lateral or radial flow into the tube bundle. Therefore, a flow distribution baffle plate has been utilized to enhance lateral or radial flow velocity by reducing vertical flow velocity. Thus, this flow distribution baffle plate is positioned a predetermined distance above the tube sheet to provide additional resistance to vertical or axial flow, thereby promoting lateral flow penetration into the bundle at the tube sheet. However, even the utilization of the flow distribution baffle plate has proved only partly successful to move the low velocity or stagnation areas toward the untubed center of the tube sheet. To solve this problem, a cylindrical flow boosting member disposed coaxially above the central region of the tube sheet has been utilized to move the low velocity or stagnation areas to the untubed center region of the tube sheet. This flow boosting member also serves to increase the lateral or radial flow velocity of the secondary fluid along the tube sheet before the radial flow enters the central region of the tubesheet.
As stated hereinabove, the particulate material will settle-out onto the low velocity or stagnation areas of the tube sheet. In this regard, the baffle plate and flow booster move the stagnation area to the center region of the tube sheet, as stated hereinabove. Thus, the particulate material and deposited sludge will tend to move to and accumulate at the center region of the tube sheet. Consequently, it would be most efficient to separate the sludge from the secondary fluid at this center region of the tube sheet so that the sludge can be efficiently removed from the secondary fluid to prevent corrosion of the tubes during operation of the steam generator. Therefore, a problem in the art has been to efficiently separate the sludge from the secondary fluid at the center region of the tube sheet. Another problem in the art has been to efficiently remove the sludge from the steam generator once the sludge is separated from the secondary fluid.
A device for control of the flow of secondary fluid around the outside surfaces of heat exchanger tubes and management of the deposition of sludge on the tube sheet of such a heat exchanger is disclosed by U.S. Pat. No. 4,704,994 entitled "Flow Boosting And Sludge Managing System For Steam Generator Tube Sheet" issued Nov. 10, 1987 in the name of Min-Hsiung Hu et al. and assigned to the Westinghouse Electric Corporation. This patent discloses a baffle plate provided to reduce vertical flow velocity, thereby tending to increase the flow velocity radially inwardly along the tube sheet. This patent also discloses a flow booster provided to enhance the flow of feedwater radially inwardly along the tube sheet. In addition, this patent discloses a central recess formed in a central region of the upper surface of the tube sheet. Moreover, the Hu et al. patent states that a blowdown pipe may be provided just above the tube sheet along an untubed center tube lane. Although the Hu et al. patent states that a central recess and a blowdown pipe may be provided for sludge management, the Hu et al. patent does not appear to disclose blowdown means having an intake orifice disposed in the recess for driving sludge from the recess and for efficiently removing the sludge from the steam generator.
A mud drum for collecting concentrated solids from the recirculating carry-over water within a nuclear steam generator is disclosed by U.S. Pat. No. 4,303,043 entitled "Sludge Collection System For a Nuclear Steam Generator" issued Dec. 1, 1981 in the name of Arnold H. Redding and assigned to the Westinghouse Electric Corporation. The Redding device provides a settling chamber or sludge collection chamber which is interposed between the recirculating carry-over water and the incoming feedwater, to intercept the recirculating water and retain at least a portion thereof in a substantially stagnant condition to permit the highly concentrated entrained solids to be deposited within the chamber. Moreover, this patent discloses that a blowdown pipe extends from the chamber to exteriorly of the heat exchanger to permit occasional or continuous discharge of the collected solids from the chamber. Although the Redding patent discloses a settling chamber and a blowdown pipe, this patent does not appear to disclose a settling chamber located on the tube sheet for collecting sludge settling on the tube sheet.
Therefore, although the patents recited herein-above disclose devices for controlling sludge buildup in a nuclear steam generator, these patents do not appear to disclose separating means located on the tube sheet for separating the sludge from sludge-bearing fluid flowing in the steam generator, blowdown means having an intake orifice disposed in the sludge separating means for removing the sludge separated from the sludge-bearing fluid, in combination with flow means disposed adjacent the separating means for directing the flow of the sludge-bearing fluid toward the separating means.
Consequently, what is needed is a system for removing sludge from sludge-bearing fluid flowing in a nuclear steam generator so that the sludge is not present therein in sufficient quantities to corrosively attack components located in the steam generator.